1. Field of the Invention
The present invention is related to a regulator, and more particularly to a regulator with a high power supply rejection ratio (PSRR).
2. Description of the Related Art
Voltage regulators are used in a variety of systems to provide a regulated voltage to other circuits in the system. Generally, it is desirable to provide a stable regulated voltage in the face of a wide variety of loads, operating frequencies, etc. In other words, a voltage regulator is designed to provide and maintain a constant voltage in electrical applications, wherein a low dropout (LDO) voltage regulator is a DC linear voltage regulator which has a very small input-output differential voltage and relatively low output noise.
A measure of the effectiveness of a voltage regulator is its power supply rejection ratio (PSRR), which measures the amount of noise present on the power supply to the voltage regulator which is transmitted to an output voltage of the voltage regulator. A high PSRR is indicative of a low amount of noise transmission, and a low PSRR is indicative of a high amount of noise transmission. A high PSRR, particularly across a wide range of operating frequencies of devices being supplied by a voltage regulator, is difficult to achieve.
For example, assume that a crystal oscillator (XO) and a digitally controlled oscillator (DCO) of an all digital phase locked loop (ADPLL) are supplied by one LDO regulator. If the clock signal generated by the XO kicks back to its supply voltage, the clock signal may kick back again to the LDO regulator's supply voltage. If a high frequency PSRR is not high enough at the frequency offset or frequency range, the kick back noise may affect the supply voltage of the DCO. To prevent the de-sensing or interference problem, high PSRR performance is very important.